A laser pick-up is generally a device for writing and regenerating data of optical information to and from an optical disk by irradiating the optical disk with laser beams from a laser diode. FIG. 1 shows a conventional laser pick-up. This laser pick-up is constructed such that diffraction grating 52, a 50% reflecting mirror 53 and toric lens 56 are positioned in the optical path. Diffraction grating 52 divides the laser beams from laser diode 51 into three beams (0 order and .+-. first orders). These beams are reflected from the 50% reflecting mirror 53 and are irradiated through an objective lens 54 to optical disk 55, thereby forming a focal spot on the optical disk 55. The beams which have formed a focus on optical disk 55 are reflected back in the reverse direction through 50% reflecting mirror 53 and toric lens 56 based on the principle of astigmatism to ultimately reach optical detector 57. Optical detector 57 detects the focus error occurred on optical disk 55 by calculating the difference between the amounts of the beams that have reached the 4 sub-parts of the detector.
Among the drawbacks of the laser pick-up arrangement described above is that 50% reflecting mirror 53 and toric lens 56 are disposed at the center of the body of the laser pick-up assembly. This arrangement makes the adjustment of the optical axis relatively difficult. In addition, the requisite number of the components is increased.
The laser pick-up arrangement illustrated in FIG. 2 was proposed to overcome these disadvantages. This arrangement includes holographic element 58 which functions as a toric lens. However, collimating lens 59 is required to make the laser beams from laser diode 51 run parallel. Moreover, using a 3-beam type pick-up arrangement to detect the tracking errors means that there will be three beams reflected after forming a focal spot on optical disk 55. Each of the beams are divided into three beams due to diffraction caused by holographic element 58 so that a total of nine laser beams reach the optical detector 57.
A further laser pick-up arrangement was designed in an attempt to resolve the problems described above. This arrangement, illustrated in FIG. 3, provides holographic element 58 and diffraction grating 52 as separate components. However, the angle between the optical axis and diffracted beams 60 from the holographic element is larger in this arrangement. The result should be that the grid gaps of the holographic element 58 are narrowed, thereby rendering the manufacture of the holographic element difficult.
Therefore, there is a need to provide a laser pick-up arrangement that minimizes the number of components and permits larger grid gaps in the holographic element to be used.